In a known image reading apparatus of the described type, optical image information picked up from an image-carrying document is projected onto an array of photoelectric transducers for producing analog electric signals representative of the image on the document. These analog electric signals representing the image on the original document are converted into digital signals by means of an analog-to-digital converter. The digital signals thus generated are supplied to a comparator circuit for comparison with a reference signal indicative of a predetermined image density for thereby producing binary signals representing the image to be reproduced.
As the reference signal used in producing the binary signals from the digital signals output from the analog-to-digital converter may utilize either a signal corresponding to a fixed image density or a signal stepwise variable within a predetermined range in accordance with a prescribed rule formulated by a "dither" pattern of the signal values dictating the stepwise variation of the reference signal. The former manner of producing the binary signals is known as the simple binarization mode and is frequently used for the reproduction of high-contrast images such as typically character images or images mostly composed of linear features. The latter manner of producing the binary signals is known as the halftone mode and is frequently used for the reproduction of halftone images such as typically photographic or pictorial images.
In the meantime, a known image reading apparatus of the described type has capabilities to adjust the density with which the images picked up from a document are to be reproduced. Such capabilities are useful for the reproduction of images clearly without respect to the density distribution of the original images on the document. The image reading apparatus having the density adjusting capabilities may have an automatic density control mode for automatically adjusting the density of the images to be reproduced, and a manual density control mode allowing the user of the apparatus to manually select the density of the images.
A conventional image reading apparatus having the automatic and manual density adjusting functions is however not fully acceptable for clearly reproducing images on a document having both character images and photographic or pictorial features or, in general, a document having irregular density distribution.